1. Field of the Invention
A certain aspect of the present invention relates to a communication method and a communication terminal for a radio communication system.
2. Description of the Related Art
Because of rapid spread of the Internet, increase in the number of information sources, increase in the volume of information, and development of the next-generation Internet in these years, research and development in next-generation radio access schemes have become very active to realize high-speed radio transmission in mobile communications. In such a high-speed radio environment, it is important to increase the uplink speed in addition to the downlink speed. For this purpose, an enhanced uplink scheme, a technology for increasing the uplink speed, has been defined by the 3rd Generation Partnership Project (3GPP), a standardization group.
FIG. 1 shows a related-art communication system employing wideband code division multiple access (W-CDMA). The communication system includes one or more communication terminals and one or more base stations. A communication terminal in W-CDMA measures a packet error rate using cyclic redundancy check (CRC) bits attached to a received downlink signal. Downlink transmission power is controlled such that a constant error rate (e.g., 0.5%) is achieved. Such a control method is called outer loop transmission power control (TPC).
FIG. 2 shows an exemplary process at a communication terminal according to the outer loop TCP.
In step S1 shown in FIG. 2, the communication terminal measures a packet error rate of a received dedicated physical channel (DPCH) based on CRC bits of the DPCH.
In step S2, the communication terminal determines whether the packet error rate is less (better) than or equal to a predetermined value (e.g., 10%). If the packet error rate is less than or equal to the predetermined value, the process proceeds to step S3. If the packet error rate is greater (worse) than the predetermined value, the process proceeds to step S4.
In step S3, because the error rate is less than or equal to the predetermined value, the communication terminal sets a transmission power control bit for the downlink transmission power of the DPCH to indicate that the transmission power needs to be decreased.
In step S4, because the error rate is greater than the predetermined value, the communication terminal sets the transmission power control bit for the downlink transmission power of the DPCH to indicate that the transmission power needs to be increased.
In step S5, the communication terminal transmits an uplink control signal including the transmission power control bit set in step S3 or S4 to the base station. Then, the base station increases or decreases the downlink transmission power according to the transmission power control bit and transmits a downlink signal (DPCH) with the increased or decreased transmission power. This kind of technology is disclosed, for example, in 3GPP TS25.214 V6.9.0.
In a radio communication system employing the enhanced uplink scheme, each communication terminal transmits data via a radio communication channel called an enhanced-dedicated physical data channel (E-DPDCH). The base station measures the interference level of the F-DPCH from each communication terminal and determines the maximum allowable level of uplink transmission power of the communication terminal based on the measured interference level to improve the throughput. Also in a radio communication system employing the enhanced uplink scheme, hybrid automatic repeat request (HARQ) is used for retransmission and combining of packets. To properly perform throughput control and HARQ control, a greater number of channels of different types are provided in a radio communication system employing the enhanced uplink scheme than in a radio communication system employing W-CDMA. The channels used in a radio communication system employing the enhanced uplink scheme may or may not include CRC bits. The transmission power of a channel with CRC bits can be properly controlled, for example, by a method as described with reference to FIG. 2. However, the outer loop TPC as shown by FIG. 2 cannot be applied without change to a channel without CRC bits (particularly, a downlink control channel described later). Theoretically, it is possible to attach CRC bits to all downlink control channels for which transmission power control is performed. However, such an approach may greatly reduce radio resources for downlink data channels.
For the above reasons, there is a demand for a device and a method that make it possible to properly control the transmission power of a downlink control channel not including CRC bits without greatly reducing radio resources for downlink data channels.